Unstable feature: copy takes arguments by value

[ghost]

This just makes our io::copy() more generic so that one can do this:

io::copy(&stream, &stream).await?;

instead of:

let (reader, writer) = &mut (&stream, &stream);
io::copy(reader, writer).await?;

This feature is only available when unstable feature flag is turned out.

cc #365

[yoshuawuyts]

@stjepang could you say more about why both versions are kept? I was thinking we keep only a single version around, but mark it unstable.

I don't think we'd be able to stabilize this way because it would break existing code wouldn't it? Or is the current trait forward-compatible?